U. Glaeser

FIGURE 27.7 Threshold of audibility in quiet approximated by Eq. (27.15).
FIGURE 27.8 Region of audibility.
where frequency f is as before given in kilohertz. The threshold of audibility computed with formula (27.15)
is shown in Fig. 27.7.
A kind of positive feedback improves the sensitivity and selectivity of the basilar membrane oscillations.
Its function can be compared with that of the so-called “reaction” used in early radio receivers to increase
their amplification and to improve their frequency selectivity. The positive feedback effect decreases as
sound intensity increases. Thus, the cochlea is less selective for intense sounds than for weak sounds. In
result, the peripheral auditory filters are nonlinear, thereby extending the overall dynamic range of the
hearing system to the range of approximately 120 dB (cf., Fig. 27.6).
Figure 27.8 shows the whole region of audibility extending from the threshold of audibility to the limit
of danger (and further up to the threshold of pain). It also illustrates two important subregions: the
speech region and the region of music. The rest of the audibility area is a reserve of the human hearing
system. Speech covers the frequency band of ca. 200 Hz to 5 kHz and the dynamic range ca. 50 dB. Music
occupies larger area, i.e., the frequency band of 50 Hz to 10 kHz and the dynamic range of ca. 70 dB.
For the representation of high quality audio it is, however, necessary to cover and reproduce practically
the whole region of audibility, i.e., the frequency band of 20 Hz to 20 kHz and the dynamic range of at
least 80–90 dB.
© 2002 by CRC Press LLC
[dB]
SPL , Lp
[dB]
SPL, Lptq
140
120
100
80
60
40
20
0
120
100
80
60
40
20
0
10 -2
-20
10 -2
Music
10 -1
Frequency, f
Threshold of audibility
10 -1
10 0
[kHz]
Threshold of pain
Limit of danger
Speech
10 0
Frequency, f [kHz]
10 1
10 1